The present invention relates to a laser recording apparatus, and more particularly to a laser recording apparatus having good tone reproducibility.
With laser recording apparatus, a record medium is raster-scanned with a laser beam which is modulated to form images on the record medium. The record medium is, for example, a photoconductive member. An electrostatic image is formed on the photoconductive member by the laser beam and then made into a copy image by a known electrophotographic process.
Various proposals have been made for reproducing tones with the use of the laser recording apparatus.
The first of these proposal is a system for subjecting the laser beam to intensity (amplitude) modulation. With this system, the intensity of the laser beam to be modulated at a constant time interval is varied in accordance with an input signal as shown in FIG. 1. Since the exposure time for each dot is definite, this system forms images with a constant resolution, whereas because the variation of the output and nonlinear modulation characteristics directly influences tone reproducibility, the system has a drawback in that it becomes increasingly difficult to obtain linear reproducibility with an increase in the number of tones.
The second proposal is a system for subjecting the laser beam to time (pulse width) modulation. This system is adapted to vary the pulse width of the laser beam to be modulated as seen in FIG. 2. Because the intensity is definite, variations of the output produce little or no influence on tone reproducibility, permitting the system to achieve substantially constant tone reproducibility. However, the exposure time which varies from dot to dot results in a drawback in that the resolution of output images tends to become unstable. Further with this system, the image recording speed is dependent on the minimum unit time which determines the pulse width, so that if it is attempted to maintain the desired recording speed while retaining high tone reproducibility, there arises a need to use a modulation frequency of the order of a gigahertz (GHz). On the other hand, a reduction in the modulation frequency entails a reduced recording speed.
According to the third proposal, the input image signal itself is processed. The systems of this type include a dot pattern system wherein each pixel is composed of a plurality of dots to reproduce tones according to the dot pattern, and a multi-level dither system wherein a simulated random number is superposed on the image signal. These systems require a period of time for processing the input signal and have the likelihood that the image quality will vary according to the tone level. The dot pattern system requires an optical system of high resolving power.
To discuss the laser source, semiconductor lasers have found wide use in recent years which have many advantages such as compactness, high efficiency, high stability and amenability to direct modulation. However, the intensity modulation system employing a semiconductor laser is unable to reproduce a very large number of tones. With reference to FIG. 3, the relationship between the exciting current of the semiconductor laser and the light output thereof changes greatly at the threshold current Ith, so that only the range between the threshold current Ith and the maximum current Imax has heretofore been used for tone reproduction. The ratio Pmax/Pth of the maximum Pmax of light output available in this range to the minimum Pth thereof is, for example, about 30 to about 50, which is about 1.5 to about 1.7 in terms of image density difference.
Additionally, the photoconductive members serving as record media vary in sensitivity from member to member and therefore exhibit different tone reproducibilities, whichever of the foregoing systems may be used. Thus, the difference between individual members must be corrected.